is Doubanjiang (Szechuan hot bean paste) an acceptable substitute in recipes calling for Doenjang (Korean fermented bean paste)?

The two have almost nothing in common, beyond being fermented and pastes (though doubanjiang is only sort of paste-like). The ingredients (broad beans and chilies versus soybeans) and the taste are very different.

That’s not to say that you’re not allowed to make a sauce for your ssam with doubanjiang, of course. But one is not a straightforward substitute for the other.

So, normally ssamjang calls for both doenjang (which is generally, but not always, not spicy), and gochujang (which is spicy). Presumably you'd be substituting your doubanjiang for both of those things, not just one of them, since it has both beans and chilis.

You'll also encounter three other differences: generally doubanjiang is looser than the Korean pastes -- it's more of a mixture than a paste, with lots of free oil. It's also both spicier and saltier than either Korean sauce.

That said, I'd bet that a sauce made with doubanjiang, green and white onion, sesame seeds, sesame oil, and honey, would actually be pretty good, even if it would taste substantially different from Korean standard. So it's a question of whether you're looking for a real Korean flavor, or just a tasty hot sauce.